A statutory body launched to protect EU citizens living in the UK will not hesitate to take the Home Office or other public bodies to court if they are found to be in breach of the Brexit withdrawal agreement, its new chairman has vowed.

“We are here and we mean business,” said Sir Ashley Fox, the chair of the Independent Monitoring Authority for the Citizens’ Rights Agreements (IMA).

In his first interview since the Guardian’s reports on EU citizens being locked up by border officials, Fox said the IMA needed to hear directly from EU citizens who feel badly treated.

“Our job is to uphold the rights of EU citizens protected under the withdrawal agreement. That’s our job and that’s what we’ll do. I am concerned at the treatment of those EU citizens of the border but I want more information before I say that we’re going to take any further action,” said Fox in an interview with the Guardian.

The IMA was set up in January under the withdrawal agreement to monitor the implementation and application of EU citizens’ rights following Brexit including the delivery of social welfare and other employment rights across various government departments.

So far it has received about 70 complaints and reached resolution on several of those about the delay and uncertainty some EU citizens experienced in obtaining health insurance cards.

Fox says the IMA has a number of powers to remedy complaints not resolved through direct engagement with a public body or government department.

“We can launch a formal inquiry into why a situation has occurred. And finally we can institute judicial review proceedings of a government body that, shall we say, ignores the evidence of an inquiry,” he said.

The IMA is also investigating whether the delays in issuing national insurance numbers because of the suspension of in-person appointments during the pandemic is breaching EU citizens’ rights.

The IMA has already intervened in a case taken on behalf of two Romanian nationals. The high court ruled in their favour over improved access to social benefits for those in the country for fewer than five years and pre-settled status.

But the so-called Fratila case has now gone to the supreme court after an appeal by the Department for Work and Pensions.

On Monday the IMA launched an official call for evidence from EU citizens to share their experiences of the EU settlement scheme, which was launched more than two years ago by the Home Office.

The IMA has identified a number of issues with the process but is looking for evidence as part of a pre-inquiry fact-finding exercise.

About 5.4m EU applications have been received in what is seen as a hugely successful process but concerns remain over vulnerable citizens who do not apply and the backlog of 320,000 applications.

Fox said the IMA would like the backlog dealt with “as quickly and effectively as possible” but warned against haste. “We don’t want mistakes made. It’s better to do this in the effective manner, and get it right.”

Fox defended a tweet over the weekend on the IMA’s account suggesting EU citizens will have “#nilpoints” like Eurovision song contest entrants if they didn’t apply for the settlement scheme before the cut-off date.

The tweet has since been deleted and an apology issued for “any offence caused”.

“To the extent that it raised awareness of the IMA and to the extent that it might cause some citizens to apply that might not have done. Then, it hasn’t done any harm has it?” he said.